1. Field of the Invention
The present invention relates to steel for a machine structural use, the free cutting characteristic, the cold forging characteristic and post-hardening/tempering fatigue resistance of which are simultaneously improved, and which therefore is used to advantage as a material for production of machine parts for use in automobiles or the like.
2. Description of the prior Art
Steel used to manufacture machine parts of industrial machines, automobiles and so forth must have a satisfactory cutting characteristic, a cold forging characteristic and a mechanical characteristic to be realized after it has been hardened and tempered, and more particularly the steel must have good fatigue resistance.
The cutting characteristic of steel is usually improved by a method in which one or more elements, such as Pb, S, Te, Bi and P, are added to the steel. Among the foregoing elements, Pb is widely used because of its significant effect of improving the cutting characteristic. However, since some elements are harmful for the human body, an exhausting facility having great size must be used in the process of manufacturing the steel. In addition, there arise a multiplicity of critical problems in recycling the steel. On the other hand, the foregoing elements obstruct the improvement in the cold forging characteristic of the steel.
As described above, the free cutting characteristic and the cold forging characteristic are usually contradictory to each other. However, the steel for a machine structural use must simultaneously have the foregoing two characteristics. In order to satisfy the foregoing requirement, graphite steel has been suggested as disclosed in Japanese Patent Laid-Open No. 51-57621, Japanese Patent Laid-Open No. 49-103817, Japanese Patent Laid-Open No. 03-140411 and Japanese Patent Laid-Open No. 03-146618.
However, inventors of the present invention have investigated the foregoing methods and found a fact that the methods cannot satisfactorily realize the characteristics required for the steel for a machine structural use. In particular, the methods cannot satisfactorily realize desired fatigue resistance.
For example, the method disclosed in Japanese Patent Laid-Open No. 51-57621 encounters a limit to refining of graphite particles, e.g., to 45 to 70 .mu.m, because only Si, Al, Ti and rare earth elements are used as elements for enhancing graphite forming. In this case, solution of graphite does not proceed quickly at the time of heating preceding quenching of the steel, thus resulting in that the obtainable fatigue resistance is unsatisfactory. The method disclosed in Japanese Patent Laid-Open No. 49-103817 does not give any specific consideration to Cr and N contents, so that the steel shown therein requires a long time for graphitization. In addition, the graphite particles are rather coarse, 38 to 50 .mu.m, hampering fatigue strength after hardening/tempering. Therefore, the process takes an excessively long time to be completed. Since the graphite forming process takes a long time, fining of graphite particles is limited. Thus, solution of graphite does not proceed quickly at the time of heating preceding quenching of the steel, and accordingly the obtainable fatigue resistance is limited. The method disclosed in Japanese Patent Laid-Open No. 03-140411 does not pay specific attention to conditions which significantly affect graphitization, e.g., hot rolling condition and graphitization annealing. Consequently, graphitization time is impractically long and graphite grain size cannot be reduced down below 28 to 35 .mu.m, thus reducing post-hardening/tempering fatigue strength. The method disclosed in Japanese Patent Laid-Open No. 03-146618 employs inadequate annealing conditions, so that the graphite grain size is as large as 21 to 26 .mu.m, failing to provide satisfaction to the demand for improvement in post-hardening/tempering fatigue strength. Thus, all these known techniques are still unsatisfactory in that they could only provide fatigue strength of 430 MPa and durability ratio of 1.2 or so at the greatest when hardened/tempered as machine part, due to coarse grain structure.